AJAX结果正常，但没有数据

I'm using jQuery to perform following AJAX request:

jQuery.ajax({
    url: "url-to-script",
    dataType: "html",
    async: false,
    success: function(data) {
        console.log(data);
    },
    error:function(){
        showMessage('Chyba', 'error');
    }
});

Script that I'm calling is returning part of HTML. It is working properly. But when I call this AJAX it will end with blank response in data. When I call it again it will return what it should and each next call is working. But the first one after page load is never working. I've no idea why.

I've also tried

if (data)
    // do something
else
    try again

But it just freeze my browser. So I don't know what is wrong.

Thanks for ideas!